In Java you have to write and compile a program only once. The Java on any platform will interpret the compiled bytecode into instructions understandable by the particular processor. However java virtual machine handles only one bytecode instruction at a time that makes execution slow. But Using the Java just-in-time compiler at the particular system platform compiles the bytecode into the particular system code. After the code has been (re-)compiled by the JIT compiler, it will usually run more quickly on the computer.
The just-in-time compiler comes with JVM and is used optionally. It compiles the bytecode into platform-specific executable code that is immediately executed. JIT compiler option should be used especially if the method executable is repeatedly reused in the code.